1. Field of the Invention
The invention is in the field of methods for forming high torque fasteners having undercut driving slots, and the resulting interim and final high torque fastener products.
2. Description of the Prior Art
A high torque fastener driving system has been in use for many years wherein a screw or bolt head is provided with a driving slot of undercut, "bowtie" configuration for cooperation with a driver tool having a driving blade with diverging side walls to prevent camming of the driver blade upwardly out of the slot and thereby permit greatly increased driving torque to be applied to the fastener. Such undercut "bowtie" fastener slots and methods for making the slots, as well as driver tools for use therewith, are shown and described in a number of prior U.S. patents issued to Rudolph M. Vaughn, including U.S. Pat. Nos. 2,677,985, 2,745,120, 2,808,087, 2,864,418, 2,949,949, 2,954,719, 2,994,354, 3,060,565, 3,103,675 and 3,388,411.
For many years, these high torque fastener recesses have been made by a milling process similar to that shown and described in the early Vaughn U.S. Pat. No. 2,677,985, issued May 11, 1954. The milling process used in manufacturing according to the present state of the art is undesirably time-consuming and expensive. Thus, this milling process requires the use of specially designed cutters which have a relatively short life and must be frequently replaced. Further, the cutter or fastener head has to be rotated about its neutral axis to produce the undercut "bowtie" configuration. The rotating cutting motion also creates a distinctive burr as the rotation takes place, and this burr adds an additional deburring operation to the fastener manufacturing process. Additionally, the fastener head has to be identified with an applicable part number, and this part identification step is subsequent and apart from the milling and deburring steps.
In addition to these problems in connection with the conventional milling process for producing undercut "bowtie" high torque fastener slots, the milling process resulted in a product having several serious deficiencies. Thus, the prior art milling process cut and interrupted the grain structure in the fastener head, thus interfering with the integrity of the body of the head material and weakening the head. Further, the prior art undercut milled slot, because of the nature of the milling operation, had side walls that are straight and unradiused in the direction from the bottom to the top of the slot, which resulted in essentially point or line contacting with the mating high torque driver blade, thus applying high stress concentrations to the head during the driving.
One prior art attempt to avoid these problems associated with the conventional milling process by forming the undercut "bowtie" slots in a series of heading steps was set forth in the Vaughn U.S Pat. No. 3,388,411, issued June 18, 1968. This Vaughn forming method required three separate heading blows, a first blow which produced a rounded head with a straight slot, a second blow which flattened the head, applied a "bowtie" configuration to the slot, and provided a double scallop raised bump arrangement along the sides of the slot; and a third blow which flattened the double scallop bumps down to provide undercuts in the flaring parts of the "bowtie" . However, it has not been feasible to utilize this Vaughn forming method in the manufacture of high torque fastener parts, for several reasons. The double scallop raised configuration is a difficult shape to make, both in making the double scallop recesses in the second punch member and in making the corresponding parts. A buildup of residue, including lubricant and/or chips of metal, characteristically packs into the punch undercuts for producing the double scallop bumps, and this results in an extremely short punch life and non-uniform product forming. Thus, this residue buildup in the second punch tends to cause incomplete final form part geometry. Additionally, after the third blow the parts will frequently still have raised head portions where the heads are supposed to be flush.